Allocation of growth between shoots and roots is a major process by which plant acclimates to various environmental conditions. It is well-known that nitrogen supply is a major determinant of this process. At high nitrogen supply, growth of above ground parts is stimulated relative to that of structures below ground, while at low nitrogen supply, root growth is enhanced (FIG. 1). This strong dependence of allocation on nitrogen supply has been demonstrated for a number of plant species and for a range of different nitrogen addition rates. Nitrogen supply will, therefore, alter the structure of plants and hence, plant resistance to various stresses such as wind and drought.
The general idea of nitrogen effects on plant biomass allocation is that at higher rates of N supply, plants sense a decreased need for nitrogen uptake and hence for root growth but an increase need for carbon uptake and hence for shoot growth. This simple relationship between nitrogen and carbon access and biomass allocation may be regulated through external cues and through sensing the levels of critical metabolites such as sucrose and glutamine within the plant.
During plant cultivation, ample supply of nutrients and in particular of nitrogen is a prerequisite for good growth. As described above, high amounts of nutrients generally stimulate growth of above ground parts more than of belowground parts and thus commercially cultivated plants, because of ample supply of nutrients; have often a high and unbalanced shoot mass fraction. There are, however, a number of situations in which plants with a high root mass fraction are superior to those with a low root mass fraction. Typically, plants precultivated for plantation outdoors or plants raised from cuttings should have a high root mass fraction to enable efficient establishment following plantation. Thus, in all situations where plants or seedlings are precultivated for later plantation, a high root mass fraction will be a positive factor for growth and establishment.
From what is described above it can be concluded that efficient cultivation of plants is incompatible with an optimal plant allocation pattern. The need for high growth rates during cultivation can only be achieved through application of high amounts of N which in turn stimulates shoot growth more than root growth and thus leads to an imbalanced allocation. Ideally, plant cultivation conditions should allow for efficient growth but still have a high root mass fraction. With current cultivation methods, this cannot be achieved.
During the last decade, a number of studies have shown that organic nitrogen compounds and in particular amino acids are important nitrogen sources for plants. These studies have demonstrated the occurrence of amino acid uptake in both field and laboratory settings and for a range of different plant species, including mycorrhizal and non-mycorrhizal plants and also for a number of crop plants, e.g. wheat, corn, barley (Lipson and Näsholm 2001). Several studies have demonstrated how absorbed organic nitrogen compounds are metabolized following root uptake and also shown how nitrogen from such sources is incorporated into proteins. Moreover, a number of studies have demonstrated that plants may use organic nitrogen compounds for growth. Thus, it is now widely accepted that organic nitrogen compounds such as amino acid may function as nitrogen sources for plants.
It is generally held that all nitrogen forms following absorption (i.e. both inorganic and organic nitrogen forms) are metabolized and thus forms a common nitrogen pool within the plant, which is available for plant growth. Thus, according to this general knowledge, all nitrogen forms absorbed by a plant root would be part of a common nitrogen pool and therefore all forms of nitrogen absorbed by a plant root should be spread evenly within the plant.
It is known from in vitro studies in RU2016510 that protein hydrolysate containing a mix of amino acids have been used for stimulating rooting of callus and plant growth in hydroponic processes. Nothing is taught about the use of a pure natural L-amino acid for stimulating root growth of plants in this patent.
AU659115 discloses a process to manufacture and the use of a fertilizer containing natural occurring L-amino acids, which are produced by enzymatic digestion of at least two proteolytic enzymes. AU659115 only discuss the advantage of using small molecules (amino acids) as nutrients to enhance the up take. Nothing is taught about the use of a pure natural L-amino acid for stimulating root growth of plants. Furthermore, nothing is taught about the simultaneously development of mycorrhiza at the same time as it stimulates growth of the whole plant in these two patents.
EP 1,284,945 describes a fertilizer suitable for plants in particular coniferous trees. An advantageous feature of the fertilizer is that it is substantially stationary thereby minimizing undesirable nitrogen leakage to the environment. The main nitrogen source of the fertilizer is the L-form of a basic amino acid or its salt, in particular L-arginine. This document is completely silent regarding stimulation of root growth and mycorrhiza development.
Thus, there is still a need for a nitrogen-containing fertilizer having the ability of stimulating root growth and/or mycorrhiza development at the same time as it stimulates growth of the entire plant.
It has been observed that plant of different species grow slowly or have slow start when planted outdoors in forest regeneration.
It has further been speculated that a high root mass fraction and a high number of root tips or fine roots would help these small plants to efficiently establish in the new growth environment
In summary there is a need of a fertilizer that can be used to increase the root mass fraction, the number of roots, root tips and the number of fine roots on plants but without compromising growth of the whole plant.